1. Field of the Invention
The present invention related to an apparatus and a method for sensing the angular position of a rotatable part e.g. a dial, wheel. The apparatus is especially intended for obtaining an electronic indication of the number wheels in a meter. More particularly, the present invention related to the non-contact angular position measurement and the low power data transmission applications. More particularly, the present invention related to the radio frequency identification (RFID) based data transmission applications. The invention may be applied to the determination of the angular position of the dials in a meter such as a water, gas or electricity meter, such demand meters are used to measure the maximum rate of flow volume or energy used averaged over a given time. In general, there are two primary requirements which are: (1) providing a rugged, reliable display of the quantity measured, and (2) the display should be nonvolatile so that the removal of power did not cause loss of the information displayed; The invention may also be applied to the auto meter reading in hash environment or flammable gas, liquid environment where the meter must be sealed and no battery or other power source allowed therewith.
2. Related Art
A previous non-contact position sensing mechanism uses a drive coil to produce alternating magnetic field and a LC resonant circuit of the rotatable member to sense the alternating magnetic field to detect the angular position of the rotatable member In the U.S. Pat. No. 5,796,250 Issued in Aug. 18, 1998 granted to Dames which including a drive coil with its axis corresponding to the axis of rotation of the rotatable member. A coil is wound around the rotatable member, and a capacitor is provided in the coil to form an LC resonant circuit. The coil on the rotatable member is wound so that application of an alternating magnetic field to the drive coil induces a resonance in the LC circuit which is constant for all angular positions of the rotatable member. The resonance of the LC circuit produces signals in a output coils which depend upon the angular position of the rotatable member. However, the use of coils to produce magnetic field needs a current source to drive the coil, the power consumed in the driving coil could be significant, especially in today's low power system. Furthermore, the resolution of the angular position detected by using this method is much lower than that of using the method of electrostatic capacity.
Another prior art, a non-contact position sensing mechanism, uses an electrostatic capacity of the relative rotatable members to detect the position of a rotary member, In the U.S. Pat. No. 5,736,865 Issued in Apr. 7, 1998 granted to Nelson. el. al. The system includes a circuit board having an arcuate array of conductor segments and a concentric pickup conductor. A rotary encoder disk has a conductive pattern which sums and couples the waveforms from selected segments to the pickup conductor; a synchronous phase detector generates a phase signal from the summed waveforms and the phase signal is detected by the zero crossing detecting circuit. A counter develops count signals used to generate the digital waveforms; the count signals are input to a latch which is clocked at the zero crossing point to latch the count value which is a digital measure of the encoder angle. However, these systems has an interference problem which is difficult to overcome, because both the array of conductor segments and the pickup conductor are provided together on a disk shape stationary part (the circuit board), the array of conductor segments and the pickup conductor are adjacently positioned. The interference with the electrostatic capacity results from interaction which occurs between the electrodes array of conductor segments and pickup conductor would not be avoided, and therefore, the output signal always includes some noise which influencing the accuracy of the output signal.
In order to solve the noise problem mentioned above, a U.S. Pat. No. 4,788,546 Issued in Nov. 29, 1988 granted to Sasaki, uses two stationary disks to separate the transmitting electrodes and the receiving electrode arranging the transmitting electrodes on a surfaces of one stationary disk and the receiving electrode on a surface of another stationary disk to isolate the driving signals of the transmitting electrodes and the pick up signal of the receiving electrode. However this configuration which provides an extra stationary disk increases the complex and the cost of product, making construction complex and expensive, because each stationary disk need to be aligned properly with two rotatable disk simultaneously.
As has been mentioned above, none of the above prior arts provided a high precision angular measurement and low power consumption system. For some applications, these solutions may be unreliable in hostile environments, or they may have insufficient resolution i.e. retaining the angular position information while removal power.